1. Field of the Invention
The present invention relates to a motor drive apparatus that performs drive control of a brushless DC motor.
2. Description of Related Art
As energizing methods for a brushless DC motor, a 120° energizing method and a 180° energizing method are common. The 180° energizing method is excellent in quietness compared with the 120° energizing method, however, requires generation of a drive waveform, and its circuit configuration becomes complicated compared with the 120° energizing method. Besides, when starting the brushless DC motor by means of the 180° energizing method, it is necessary to start rotation of the rotor irrespective of a rotor position by means of some approach and to switch to energizing control in accordance with the rotor position on the way.
For example, when starting a brushless DC motor that has many poles (12 poles or 16 poles), a method is commonly employed, in which the rotor is forcibly started to rotate by means of an energizing signal that has a predetermined magnetization pattern (forced commutation pattern) and switched to the 180° energizing control on the way.
In the meantime, as an example of a conventional technology relevant to the above description, there is JP-A-2008-148379.
On the other hand, in a case where the above forced commutation method is employed as an approach for starting a brushless DC motor that has a small number of poles (2 poles or 4 poles), there are problems: (1) difficult matching of a forced commutation frequency; (2) a defective start occurring because of a rotation ripple during a forced commutation period; and (3) a large noise caused by the rotation ripple during the forced commutation period.
Therefore, as the approach for starting the brushless DC motor that has the small number of poles, the above forced commutation method is not employed, but a method is employed, in which the rotor is started to rotate by means of the 120° energizing method, which is switched to the 180° energizing control on the way. However, in the case where the method is employed, there is a problem that a noise (motor start noise) is large when rotating the motor by means of the 120° energizing method.